Thesis Defense: Tunahan Aytas

Title

Kinetics of the cementitious systems with the incorporation of unreactive 
industrial byproducts and waste materials

Abstract

The utilization of industrial waste materials as Supplementary Cementitious Materials (SCMs) has gained significant attention in recent research. However, while ordinary Portland cement adheres to strict industrial standards, alternative SCMs often exhibit considerable local and seasonal variations. In order to achieve sustainable cement replacement, it is imperative to develop tools and methods for assessing the suitability of materials as SCMs. To accomplish this, it is crucial to gain a comprehensive understanding of cement hydration kinetics, which involves the simultaneous dissolution of precursors and precipitation of hydration products. Nevertheless, quantifying the rates of dissolution and precipitation poses a challenge.
 
This thesis primarily focuses on dissolution kinetics, initially establishing a database of crystalline and amorphous silica (quartz and silica fume) dissolution rates under various conditions (concentration, temperature, pH, pressure). The database is supported with experimental data on dissolution rates in sodium hydroxide solutions, which mimic the high pH of cement pore solution. Based on this data, predictive models, random forest and artificial neural network models, are constructed to estimate dissolution rates.
 
Furthermore, another predictive model is proposed for the composition of hydrated phases in blended Portland cement mixes by utilizing the dissolution rates of individual mineral phases found in copper and slag slags at pH 13, in combination with the Parrot-Killoh model. The assemblage of reaction products is predicted through thermodynamic simulations, based on the quantities of consumed precursors. A case study is conducted using a crystalline ladle furnace slag, and the performance is evaluated by comparing predicted and measured values of bound water and portlandite content. The model also suggests the replacement of gypsum in order to counterbalance the rapid dissolution of reactive alumina-rich phases, leading to improved strength, modified reaction mechanisms, and reduced permeable pore content.
 
Subsequently, the thesis investigates precipitation kinetics. A 3D-printed flow reactor is employed to calculate the precipitation rate of calcium silicate hydrate (C-S-H). This flow reactor offers a rapid and practical approach for measuring these kinetics. The precipitation rates of C-S-H are measured using three different mineral phases as nucleation surfaces: calcite, diopside, and fayalite. Higher precipitation rates are observed on calcite and diopside surfaces compared to fayalite surfaces, indicating the potential use of diopside as a filler material in concrete mixes. Factors such as higher pH, lower flow rate, and increased supersaturation also contribute to higher precipitation rates.
 
Throughout this thesis, a range of methods, including data-driven approaches, experimental kinetics rate measurements using batch and flow systems, and thermodynamic simulations, are employed to assess the impact of alternative SCMs in blended systems. These methods enable the exploration of dissolution and precipitation kinetics in blended systems separately.

Thesis Advisor

Elsa Olivetti, Esther and Harold E. Edgerton Associate Professor of Materials Science and Engineering, MIT